1. Field of the Invention
The present invention relates to a method of refining a lubricant for use on a magnetic recording medium, the lubricant that is refined by the method, and a magnetic recording medium that uses the lubricant.
2. Description of the Related Art
A magnetic recording medium is commonly used in a fixed magnetic recording medium device as a data recording medium of a computer. The fixed magnetic recording medium device includes a driving mechanism of the magnetic recording medium, a driving mechanism of a magnetic head, a stopping mechanism of a magnetic head, a data transfer and control mechanism, and one or more magnetic recording media mounted on the magnetic recording medium device. Significant tendencies in the development of the fixed magnetic recording medium in recent years are high recording density, large capacity, and high data transfer speed.
In an operation scheme of a conventional fixed magnetic recording medium device, the magnetic head floats when the magnetic recording medium is rotating, and the magnetic head contacts with a surface of the magnetic recording medium when a driving motor to rotate the magnetic recording medium stops. The operation scheme is called a CSS (contact start and stop) scheme. When the magnetic recording medium begins to rotate and stops in the CSS scheme, the magnetic head slides and rubs with the surface of the magnetic recording medium. To protect the magnetic layer when rubbing with the surface of the magnetic recording medium, a protective layer is laminated, and to improve a surface lubrication performance, a lubricating film is laminated.
A rotating speed of the magnetic recording medium increases from about 5,400 rpm (revolutions per minute) to the range between 7,200 to 15,000 rpm. As a result, a spin migration phenomenon grows significantly, in which the lubricant on the magnetic recording medium surface moves towards an outer periphery due to a centrifugal force and scatters. A thickness increase of the lubricant in the peripheral region due to the spin migration causes a problem of sticking. Higher speed of rotation gives rise to heating, which causes evaporation of the lubricant. The evaporation of the lubricant decreases thickness of the lubricant, which may cause wear of the protective layer. A head crack may happen in a worst case.
In order to suppress the spin migration, the lubricant film is necessarily formed with a uniform thickness and with stability on a surface of the protective layer, and high adhesiveness and strong bonding between the lubricant film and the protective layer are important. The evaporation of the lubricant is also prevented by enhancement of the adhesiveness and bonding strength. To enhance the adhesiveness and bonding strength, a perfluoropolyether lubricant having an end group of hydroxyl group or piperonyl group is used. Examples of such lubricant are “Fomblin Z DOL” and “AM3001”, which are commercially available from Ausimont K. K., Tokyo, Japan. Most of the perfluoropolyether lubricants used at present have an average molecular weight in the range between 1,000 and 10,000. If the molecular weight is too low, lubricating performance and heat resistance degrades; on the other hand, if the molecular weight is too high, adhesion tends to occur. However, these lubricants deteriorate durability to the spin migration at the high rotating speed in the range between 7,200 to 15,000 rpm, although the adhesiveness and bonding strength of the lubricant are enough at the rotating speed of about 5,400 rpm that is employed.
Accordingly, development is being made for a material having a functional group with strong polarity that is introduced into a perfluoropolyether lubricant in order to enhance adhesiveness and bonding. Japanese Unexamined Patent Application Publication Nos. H11-172268 and 2001-6155, for example, disclose that introduction of an amine end group significantly enhances bonding strength. Synthesis of these end-group-modified materials, however, requires long time and dedicated equipment, which arouses apprehension for increased cost for a manufacturer of the magnetic recording media to synthesize these materials.
Commercially available lubricant “Fomblin Z tetraol” from Ausimont K. K., Tokyo, Japan is a perfluoropolyether having a molecular structure in which two hydroxyl groups are introduced at each of the both ends. The lubricant is superior to “Fomblin Z DOL” from a viewpoint of the adhesiveness and bonding strength with the protective layer. The lubricant is known to contain a coexisting lubricant that has a structure including only one hydroxyl group at the end group. A structure including only one hydroxyl group at the end group is the structure of “Fomblin Z DOL”. A coexistence of this lubricant component causes deterioration of adhesiveness and bonding strength from an expected performance.
Ionic impurities contained in the lubricant cause corrosion on the magnetic recording medium. The ionic impurities induce dissolution of cobalt used in a magnetic layer. The cobalt ions act as a catalyst and dissolve perfluoropolyether lubricant molecules. Therefore, the ionic impurities are to be eliminated as thoroughly as possible. However, because the lubricant inevitably contains ionic impurities that are used in the synthesis process of the perfluoropolyether lubricant, the ionic impurities must be removed before they are applied to the magnetic recording medium.